Method of producing a fibrous product and a fibrous product

ABSTRACT

The present invention relates to multi-layer fiber products and a method of manufacturing these kinds of products. The present product comprises a first layer consists mainly of natural fibers and a second, heat-sealing layer located on top of the first layer. The heat-sealing layer consists mainly of synthetic thermoplastic fibers or particles. According to the present method, the heat-sealing layer is brought onto the first layer already during the web forming process, the first and the second layers being formed and joined together in a foam forming process. With the present invention, it is possible to decrease the amount of plastic materials in packaging materials having heat-sealing properties.

FIELD OF THE INVENTION

The present invention relates to a method according to the preamble ofclaim 1 of manufacturing a fiber product.

In a method of the present kind, a base layer comprising mainly naturalfibers is formed in a web forming process and a heat-sealing layer isbrought onto the base layer in order to prepare a multi-layer fiberproduct.

The present invention also relates to a fiber product according to claim20 and a use according to claim 26.

DESCRIPTION OF RELATED ART

Consumers today demand increasingly environmentally friendly packagingsolutions. For this reason, for many retail chains and brand ownerspaper and paperboard are preferred packaging materials because suchproducts are obtained from renewable raw materials and are readilyrecycled. Many packages that require for example good barrier propertiesagainst oxygen, water and greases are provided with layers of materialsthat have heat-sealing properties. For this reason, paper-plasticlaminates are frequently used for packing candies and bakery products,such laminates being manufactured by extrusion coating of paper or bylaminating paper together with plastic films by using cold or hot glue.

Due to the bonding of extrusion films to paper or paperboard substratesor to a required increase in the strength of the laminated film, theproportion of plastics in these film laminates is often tens of percent,and therefore the materials are difficult to pulp, and recycling isfrequently based on heat recovery by combustion. For example, in theextrusion coating of paper the minimum thickness of plastic film isusually about 15 um, since a film thinner than the indicated thicknessmakes it difficult to obtain sufficient adhesion of the film to thepaper.

On the other hand, by lamination it is difficult to attach films whichhave a thickness less than 20 um, as the film must be self-supporting inthe lamination process.

FI 126474 B discloses a product comprising a natural fiber matrix, intowhich thermoplastic fibers can be incorporated as reinforcing fibers,thus conferring heat sealing properties of the product. However, theamount of such reinforcing fibers mixed to the matrix must be relativelyhigh in order to provide good enough heat-sealability.

For example due to the above mentioned short-comings of the presentconversions processes and materials, the current heat-sealing papermaterials comprise relatively large amount of plastics which impairsrecycling of the materials. In addition, the manufacturing of suchmaterials requires in principle one or more extra process steps, whichincreases the costs.

Further, heat-sealing products based on plastic films are notbreathable. However, many packaged products, for example powderyproducts, require breathability from the package due to theeffectiveness of the packaging line. For many products, such as textilesand furniture, aeration or ventilation during storage and transport isdesirable. Bakery products, such as bread, benefit from breathablepackages because that will shorten delivery times by eliminating theneed for cooling of the products before packaging.

SUMMARY OF THE INVENTION

It is an aim of the present invention to eliminate at least some of theproblems associated with the art and to provide a new type ofheat-sealing product and a method to manufacture a heat-sealing fiberproduct.

In particular, it is an aim to improve the recyclability of heat-sealingpaper-type materials by reducing the amount of thermoplastic materialsincorporated into the products and by increasing the cost-effectivenessof the manufacture of recyclable packaging materials.

A further object of the present invention is to provide a method bywhich novel heat-sealing and air permeable end products can be produced.

According to the present invention, onto a substrate layer, mainlycomprises natural fibers, is formed a heat-sealing layer comprisingmainly synthetic fibers or particles, especially long synthetic fibers.

It has been found in the invention, that such a separate heat-sealingfiber layer enables a considerably much thinner total layer thicknessand thus a smaller total amount of synthetic material. On the otherhand, by the invention materials are provided which can be joined with aheat-seal that is typically stronger than that obtained for conventionalextrusion coated or laminated films and also a stronger heat-seal thanthat obtained for products where the same amount of synthetic fibers orparticles is evenly incorporated into the base layer.

In the method according to the present invention, a layer, in particulara heat-sealing layer, is brought onto the substrate layer already at theweb forming step, whereby the heat-sealing fiber layer is stronglyattached to the substrate layer without the need for any entirelyseparate process steps or lines.

According to the present invention, preferably at least one of thelayers, in particular at least one substrate layer or at least oneheat-sealing layer, or both, are formed by foam forming.

More specifically, the method according to the present invention ismainly characterized by what is stated in the characterizing part ofclaim 1.

The product according to the present invention is, in turn,characterized by what is stated in the characterizing part of claim 20.

The use according to the present invention is defined in claim 26.

Considerable advantages are obtained by means of the present invention.

In particular, by means of the invention it is possible significantly toreduce the proportion of plastic of the novel materials with up to 75%,compared to paper-plastic laminates commonly used as packagingmaterials. This is due to the fact that a fiber or particle richheat-sealing layer can be produced as a very thin structure whichattaches at the web forming step firmly to the substrate layer of theproduct. Thus, the invention enables the manufacture of multi-layermaterials that are easier to recycle. Since the invention removes oneseparate refining process from the production chain, the invention alsoallows a manufacture of cost-effective heat-sealing materials.

Further, compared to existing plastic imitating products in whichsynthetic thermoplastic fibers have been mixed evenly to the whole fibermatrix, the heat-seals according to the present invention have beenfound to be up to 60% stronger with the same starting materials andtotal amount of material. This enables the use of even smaller amountsof plastic materials, thus enabling easier recycling of materials andyet the manufacturing of strong seams, which is essentially important inthe packaging industry.

Compared to products manufactured by lamination or extrusion coatingmethods, an advantage is obtained that the product is breathable, sincein addition to the base layer the heat-sealing layer of the fiberstructure may be implemented as breathable structure as well, unlikeself-contained films.

Preferably, both the base layer and the heat-sealing layer are formedand attached to each other in the foam forming process when thedry-matter content of the product is still low, for example 35% or less.The seal of the layers can be strengthened by adding a binder to thelayers or at least to their interface zone. The used binder ispreferably heat-sealable itself when the need of synthetic fibermaterial further decreases. On the other hand, a sufficiently stronginterconnection can be ensured by calendaring in a sufficient pressureand temperature.

The thickness of the heat-sealing layer is typically considerablysmaller than the one of the base layer, heat-sealing layer having agrammage of 15 g/m² or less, preferably 10 g/m² or less, more preferably5 g/m² or less.

An absolute thickness of the heat-sealing layer is preferably 10 um orless, for example 2-10 um, which is significantly less than what ispossible to obtain in conventional heat-sealing products.

In the following, the preferred embodiments of the present inventionwill be examined more closely on the basis of the companying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a cross-sectional view of the structure of a double layermulti-layer fiber product according to one embodiment of the presentinvention.

FIG. 1B shows a cross-sectional view of the structure of a three layermulti-layer fiber product according to one embodiment of the presentinvention.

FIG. 1C shows a cross-sectional view of the structure of a multi-layerfiber product according to one embodiment of the present invention, inwhich structure the base layer and the heat-sealing layer are partiallymixed with each other in order to strengthen the bonding between thelayers.

FIGS. 2A and 2B show flow charts of the manufacturing method accordingto the present invention according to two embodiments of the presentinvention.

FIG. 3 shows the results of the strength tests described in example 1 asa graph.

EMBODIMENTS

The term “layer consisting mainly of X” (or “mainly comprises”) in thepresent context means that said layer comprises at least 50 weight-% ofmaterial X.

The term “heat-sealing” refers to a structure and material which iscapable of forming a permanent seal under the influence or heat andpressure at least with another similar structure and material.

The terms “web forming process” and “forming” stand for the formation ofweblike fiber-based structure by a wet-laid process where raw materialscontained in the final product are introduced onto the wire in the formof an aqueous slush or a foam, they are shaped or formed into a fibrousweb which is dried in a drying section of a paper or paperboard machineto provide the final fibrous matrix. By “foam forming” is meant inparticular the method as disclosed in FI126474.

The term “long” when used for synthetic fibers generally means fibershaving a length of at least 1 mm.

The term “multilayered web” of “multilayered fibrous product” stands fora web or fibrous product that is formed by at least two overlappinglayers of fibers. There can be a plurality of such overlapping layers,typically 2 to 10, in particular 2 or 3. Preferably, at least one oflayers forming the surface of the multilayered web or product is formedby a heat-sealing layer. In one embodiment, the layer adjacent to theheat-sealing layer is a substrate layer which typically does not haveheat-sealing properties by itself. In addition to the fibrous layersthere can be also a surface layer conferring preselected properties, forexample of hydrophobicity, to the layered products.

The “fiber lengths” of “average fiber lengths” are expressed in terms oflength weighted average. Length weighted average fiber length iscalculated as the sum of individual fiber lengths squared divided by thesum of the individual fiber lengths.

As will appear from the above, in the manufacture method of a preferredembodiment, the heat-sealing layer of a paper material is brought ontothe product already at the manufacturing stage to provide a multilayeredproduct. Such a multilayerered fibrous product it typically composed ofa first layer, for example a base layer, mainly comprising naturalfibers, such as chemical pulp, mechanical pulp, semimechanical pulp,recycled fibers or any combination of these, and a second layer, i.e.the heat-sealing layer, which mainly comprises synthetic fibers,particles or combinations thereof, that is formed onto the first layerin the web forming process.

In one embodiment, a method of manufacturing a multilayered fibrousproduct which comprises at least two overlapping layers is achieved in aweb forming process, in particular by foam formation. In the embodiment

-   -   a first layer containing mainly of natural fibers is provided,        and    -   a second layer containing synthetic fibers or particles        provided, the first and the second layers being arranged in        overlapping relationship. The second layer is capable of        conferring properties of heat-sealing to the fibrous product.

In one embodiment,

-   -   the first layer is formed into a web from a first slush having a        first composition, and    -   the second layer is applied onto the web formed by the first        layer from a second slush having a second composition to form a        multilayered web.

The first and the second compositions differ from each other for examplewith regard to the content of at least one component selected fromfibers, polymers, additives and water.

Preferably at least one, in particular both of the compositions containfoaming agents.

In one embodiment, the first and the second layers are formed and joinedtogether in a foam forming process using a multilayer headbox.

According to one embodiment, the present multi-layer fiber product ismanufactured using foam forming technology at least for the productionof the base layer, and preferably also for the production of theheat-sealing layer and interconnecting of the layers. In general, thetechnology is described in publications FI 126474 B and FI 126092 B. Thefoam forming enables even distribution of long synthetic fibers also inthin layers, and thus a high quality end product.

In one embodiment, foam forming of the present multi-layer product iscarried out using a single forming unit to which the foam streams arefed in layers or in sections.

The forming unit can be horizontal or vertical. For example, in atwo-layered web, one foam stream forms the base web. The dry-matter ofthat stream comprises mainly natural fibers. The other forming layerwhich forms the sealing layer comprises mainly thermoplastic fibers.

Generally, by foam forming high consistency of the furnish (high drymatter content of the slush) can be maintained. Typically, in thepresent technology, the consistency in the headbox is greater than 0.1%by weight, in particular greater than 0.5% by weight and up to about 3%by weight.

In a preferred embodiment, a part of the starting material is firstformed into a water-containing, planar base web which comprises a firstwater phase and a first fiber phase mainly comprising natural fibers. Inparticular, the web is formed by foam forming, as described below bydispersing the fibers in a foam or a foamble liquid of water and atleast one foaming agent. The heat-sealing layer is attached to the baseweb as a partial web or as a dispersion, in particular as a foamedpartial web or as a foamed dispersion, comprising a second water phaseand a second fiber or particle phase mainly comprising synthetic fibersor particles or combinations thereof, and by allowing the syntheticfibers and any particles of the second fiber phase to infiltrate atleast partially between the natural fibers of the first fiber phase.

The resultant moist multilayered web is dried to remove the water,whereby the natural fibers and synthetic fibers form a layered fibermatrix together. In the same connection, one or more binders can beapplied to the first and/or second aqueous phase, or over the whole web,whereby also the binder infiltrates to the desired layers. Preferably,the binder is selected so that it is compatible with the fibers andparticles of the heat-sealing layer.

Generally, the ratio of the velocity of the foam stream to the velocityof the wire (upon which possible already a first layer of a first slushhas been deposited) is in the range of 0.1 to 2.5.

In one embodiment, the ratio is not equal to 1. In that embodiment,there will be a mixing of the layers upon the wire to allow fordevelopment of interlayer bonding.

In one embodiment, the ratio is greater than 1 and up to 2.5.

In one embodiment, foam forming comprises forming a foamed dispersion bydispersing fibers in a foam or foamable liquid of water and at least onefoaming agent to obtain fiber-foam comprising typically 0.1 to 15%, forexample 0.5 to 10% by weight of at least one foaming agent. The thusformed fiber-foam is then conveyed to a foraminous support, such as awire, and liquid is drained trough the foraminous support to form asheet.

The foamining agents for producing foams are selected from surfaceactive agents, which enable the formation of a foam. Thus, typically,the foaming agent is selected from water-soluble foaming polymeric agentand water dispersible foaming polymeric agents and combinations thereof.The foaming agent can be selected from water soluble glycans, waterdispersible glycans, water soluble hydrophilic polymers and waterdispersible hydrophilic polymers and combinations thereof. Preferablythe water soluble glycans and water dispersible glycans are selectedfrom polysaccharides and derivatives thereof. The water solublehydrophilic polymers and water dispersible hydrophilic polymers can alsobe selected from poly(vinyl alcohol)s and poly(vinyl acetate)s andcopolymers thereof.

In one embodiment, where the fibers do not form hydrogen bondings, thefoaming chemicals act also as a binder.

In one embodiment, in particular carried out using separate headboxes,it is possible first to dry the layer formed from a first slush from afirst headbox to remove a portion, typically a major portion, of thewater before a second layer is deposited from a second headbox on thestill wet first layer. Thus, in one embodiment,

-   -   a first layer is formed into a web having a first solids        content,    -   the web is subjected to drying to increase the solids content of        the web to provide a modified web having a second solids content        (second dry matter content), and    -   a second layer is applied onto the modified web.

Similarly, in particularly using separate headboxes:

-   -   a first layer is formed into a web having a first solids        content,    -   the web is subjected to pressing in a press section, to cylinder        drying in cylinder section or to a combination thereof, in order        to increase the solids content of the web to provide a modified        web having a second solids content (second dry matter content),        and    -   a second layer is applied onto the modified web.

The first layer has initially a high water content, on the order of thatof the first water composition used for web forming, and it is dried toa higher solids content (dry matter content) so as to form a modifiedweb prior to deposition of the second water composition. For example.The first layer is dried to a second solids content of 15 to 35% byweight, in particular 20 to 35% by weight, to form a modified web beforeapplying the second layer on the web.

In one embodiment, drying of the web and in particular of themultilayered web is carried out by a non-contact drying.

Drying is, for example, carried out by at least one method selected fromthe group consisting of vacuum drying, hot air drying,through-air-drying, IR drying, cylinder drying and combinations thereof.

Drying is typically carried out until a final moisture content of lessthan 20% by weight, typically less than 15% by weight, in particularless than 10% by weight is reach.

In one embodiment, the particles (including fibers) of the layers have asize greater than the pore size of the wire. This allows for the use ofvacuum for enhancing water removal from the web.

The application of a binder can be carried out onto the partial web ormulti-layer web, i.e. separately onto the webs of the base layer web andheat-sealing layer before bonding of these together, or only after thebonding. The structure can be strengthened by applying binder at theinterface of the layers, either on one or both partial webs, typicallyonto their contacting surfaces, before the joining the webs together.

In one embodiment, both the base layer and the heat-sealing layercomprise the same thermoplastic binder, suitably evenly spread upon orin the layers.

The hydrophobic properties of a product as disclosed herein can befurther improved. Thus, in one embodiment, on the opposite surface ofthe heat-sealing layer of the base layer is arranged a layer havinghydrophobic properties. One example is a surface-lacquer layer.

Hydrophobic properties can be conferred to a surface layer of themultilayered product by using hydrophobic agents, such as alkyleneketene dimer (AKD), alkenylsuccinic anhydride (ASA) and binders incombination with cross-linking binding agents.

Regarding the fibrous and particulate materials of the layers, it can benoted the following.

In the present context, in particular, the natural fibers are sourcedfrom cellulose or lignocellulose raw materials, for example, by usingchemical or semi-chemical pulping or defibering. The fibers can also bemechanical pulp fibers or recycled fibers. Especially, the naturalfibers refer to natural fibers and their mixtures described moreprecisely in publication FI 126474 B.

As raw material of the cellulose or lignocellulose fibers, woodmaterials or vegetable materials, such as annual or perennial plants canbe used. In particular, deciduous tree material (obtained from birch,aspen, poplar, alder, Eucalyptus or mixed tropical hardwood, forexample) or, in particular, coniferous tree material is used as araw-material. Examples of the latter include wood obtained from spruceor pine.

The cellulose or lignocellulose fibers can be refined.

In one embodiment, non-refined cellulose or lignocellulose fibers, inparticular cellulose fibers, are employed.

In one embodiment, the base layer (substrate layer) consists mainly ofnatural fibers. In one embodiment, the base layer (substrate layer)comprises more than 50% by weight and up to 100% by weight, of naturalfibers calculated from the total weight of the fibers of the base layer(substrate layer).

In one embodiment, the base layer (substrate layer) may comprise 1-49%,typically about 1 to 30%, for example about 1 to 20% by weight of otherfibers, especially synthetic fibers.

Further components, for example binders or other additives, andpotentially other components required by the product or themanufacturing process, can also be incorporated. The amount of suchcomponents are in general about 0.1 to 30 parts by weight per 100 partby weight of the layer.

Examples of binders include natural binders and biopolymers, such asstarch and modified starch and starch derivatives, chitosan, alginates,and synthetic binders, such as vinyl acetate and vinyl acrylate latexand polyurethanes and SB-latexes, as well as mixtures thereof andvarious copolymers, particularly copolymers of synthetic binderpolymers.

The heat-sealing layer may comprise up to 100%, for example 50-100%,such as 51-99%, by weight of thermoplastic fibers and/or particles, suchas polylactide (PLA), glycolic acid polymer, polyolefin, polyethyleneterephthalate, polyester, polyamide, polyvinyl alcohol or bicomponent(bico) fibers or particles. Particularly, the material may be PLA,polypropylene (PP), polyethylene (PE) or bicomponent fibers. Also otherbiopolymers, such as polybutylene succinate (PBS), are potentialpolymers for these synthetic fibers or particles. One of the preferredexamples is PLA-PBS multi-layer synthetic fibers or particles.Especially thermoplastic fibers can be biodegradable thermoplasticfibers.

As a raw material of the heat-sealing layer, for example up to about 50%by weight, for example about 0.1 to about less than 50% by weight (forinstance 1 to 49% by weight), other fibers, such as long reinforcementfibers, especially viscose, or natural fibers or other thanthermoplastic synthetic fibers can also be used. Thus, for example,cellulose or lignocellulose fibers of the above-described kind (andrefined or preferably non-refined) can be incorporated into theheat-sealing layer. Typically the amount of such fibers is up to 40% byweight, in particular about 5 to 30% by weight.

The synthetic fibers of the heat-sealing layer may have a linear densityof, for example, 10 dtex or less, preferably not more than 5 dtex, suchas 1-5 dtex. The average length of the long fibers may be, for example,1-50 mm, preferably 1-20 mm, such as 3-12 mm. Typical PLA fiberssuitable for use have a length mass of 1.7 dtex and a length of 6 to 12mm.

Binders and/or other additives required by the product or themanufacturing process can also be incorporated. The amount of suchcomponents are in general about 0.1 to 30 parts by weight per 100 partby weight of the layer.

Examples of binders include natural binders and biopolymers, such asstarch and modified starch and starch derivatives, chitosan, alginates,and synthetic binders, such as vinyl acetate and vinyl acrylate latexand polyurethanes and SB-latexes, as well as mixtures thereof andvarious copolymers, particularly copolymers of synthetic binderpolymers. In one embodiment, binders which are capable of heat-sealingare used. In one embodiment, binders which are capable of conferringproperties of elasticity to the heat-sealing layer, such aspolyurethanes, are employed.

The interconnection of the layers may be carried out or it may beimproved also by heat, for example by calendering the multi-layer web atleast at 90° C. and preferably above 130° C.

In one embodiment, the aqueous composition (“first slush”) for the firstlayer contains in addition to natural fibers, and optionally syntheticfibers and/or particles, also 0.1 to 15% by weight, calculated from thesolids content, of an additive selected from surfactants, binding agentsand combinations thereof.

In one embodiment, the aqueous composition (“second slush”) for thesecond layer contains in addition to synthetic fibers and/or particles,and optionally other fibers, such as natural fibers, also 0.1 to 15% byweight, calculated from the solids content, of an additive selected fromsurfactants, binding agents and combinations thereof.

As will appear from the above, in one preferred embodiment, the firstlayer is “non-heat-sealing”, i.e. incapable of heat sealing againstanother similar layer, whereas the second layer is capable of heatsealing against another layer.

Turning next to the drawings which illustrate various embodiments moreclosely, it can be noted that FIG. 1A shows a basic structure with abase layer 12 (which also can be referred to as “substrate layer”) and aheat-sealing layer 14 arranged onto the first surface of the base layer.FIG. 1B shows an embodiment in which also onto the second, oppositesurface of the base layer is arranged a functional surface layer 16.This second surface layer 16 can be for example a hydrophobic layer inorder to provide the final product with properties of both heat-sealingand improved moisture-proofing.

In general, the heat-sealing layer 14 is thinner than the base layer 12.Typically its grammage is 10 g/m² or less, preferably 1-5 g/m², such as2-4 g/m², with the grammage of the base layer being preferably 20-100g/m², for example 30-70 g/m², in particular 20-60 g/m² or 20-40 g/m². Inone embodiment, a 70 g/m² multilayered product (a sheet or web) wasproduced with a 10 g/m² heat-sealing layer overlapping a 60 g/m² fibrousbase layer. In another embodiment, a multilayered product having agrammage of 30 g/m² for the base layer and 3 g/m² for the heat-sealinglayer were produced.

As discussed above, the base layer may contain 50-100%, for example51-99%, by weight of natural fibers, typically cellulose orlignocellulosic fibers.

In an embodiment, a multilayered structure as shown in the drawings ismanufactured by a paper or paperboard machine by using a multilayerheadbox configured for foam forming.

Referring to the FIG. 2A, it can be noted that, according to oneembodiment, the partial web forming the base layer and the partial webforming the heat-sealing layer are formed in steps 21A and 21B in a foamforming process from different masses using a multi-layer headbox of apaper machine, after which a multi-layer web is formed from the partialwebs in step 22.

The multi-layer web is further dried in step 23 in order to prepare amulti-layer fiber product. Optionally, a binder can be applied to thepartial webs in steps 21A and 21B. In addition or optionally, a bindercan also be applied to the multi-layer web. The multi-layer web can alsobe calendered in order to improve the interconnection of the layers.

Referring to the FIG. 2B, according to one embodiment, the base webforming the base layer is formed in step 26 by foam forming, after whichonto the surface of the web forming the base layer are appliedthermoplastic synthetic fibers or particles in step 28, still during theweb forming phase, in order to produce a multi-layer web. Before this,also a binder can be applied to the base web in step 27. Finally, themulti-layer web is dried in order to produce a multi-layer fiber productin step 29.

By foam forming, in particular carried out in connection with the webforming, it is possible to adjust the mixing between the differentlayers in order to achieve the required layer strength.

According to one embodiment, visualized in the FIG. 1C, the fibers ofthe base layer 12C and the heat-sealing layer 14C are mixed in theinterface zone of the layer such way that the interface of the layersbecomes “sliding”. The thickness of the interface zone 13C can be forexample 5-50% of the thickness of the heat-sealing layer 14C, forexample 0.5-3 um. The mixing strengthens the adhesion and thus theentire product and especially the heat-seams produced from the product.A corresponding mixing can also be produced possibly to the other layerinterfaces of the product. If necessary, the multi-layer structure canalso be produced by foam forming such way that layers are notsubstantially mixed with each other.

The base layer 12, 12C, is referred at this unit but in general also thebase layer may comprise several sublayers. According to one embodiment,the base layer 12, 12C, as well the heat-sealing layer, compriseshowever substantially a single layer and has a homogeneous fibercomposition.

Example 1. The Effect of the Layer Structure on Heat-Sealability

A sheet mold was used to manufacture sheets with a first layer on thewire-side consisting of 100% cellulose fibers and the advance layer,i.e. a heat-sealing layer, consisting of a blend of cellulose fibers andpolylactide (PLA) fibers. A binder was applied on the sheet on the sideof the first, non-heatsealing layer. The layers were calendered attemperature of 90° C. using one nip and a pressure of 10 bar.

The sheets were seamed using a force of 800 N, at temperature of 210° C.and a heat sealing time of 0.5 s. The strength of the seams weremeasured with a horizontal traction device using samples strips having awidth of 50 mm. The strips were cut from the sheet in the crossdirection such way that the seams were orientated in machine direction.The paper strips drawn speed was 20 mm/min and draw gap 100 mm. 3-4parallel determinations were carried out on each sample.

The same amount of total material were used in samples (1a, 2a, 3a, 4)and reference samples (1b, 2b, 3b), however, such way that in thesamples the synthetic fibers were mixed only to the part of the amountof the cellulose fiber. Thus, a layer structure more similar to thatdescribed above was generated. In the reference samples the syntheticfibers were evenly mixed with the whole amount of the cellulose fiber.

Further details on materials and results are shown in Table 1 and FIG.3.

TABLE 1 Measurement of seam strength Pulp fraction to Total which PLAPLA % Seam Strength grammage has been PLA of total strength growthSample g/m² mixed, % g/m² weight N/m % 1a 96 50 15 16 191 62 1b 94 10015 16 118 2a 109 50 30 27 492 151 2b 112 100 30 27 196 3a 62 50 15 24417 60 3b 62 100 15 24 261 4  78 17 5 6 200

As will appear from the results of the samples (1a, 2a, 3a, 4) andreference samples (1b, 2b, 3b), respectively, the seam strengthincreased with at least 60% when the synthetic fibers were layered onone side of sheet, compared to a situation where PLA fibers were evenlydistributed throughout the entire structure of the sheet.

Sample 4 shows that even a thin (5 g/m²) heat-sealing layer on top of abase layer of natural fibers produces a strong seam. Thus, the presentsealing strengths, sufficient for practical applications, can beachieved with the structures described above using significantly smalleramounts of synthetic polymer than when mixing the synthetic polymerevenly into the entire fiber product.

INDUSTRIAL APPLICABILITY

The product according to the present invention can be used for exampleas a heat-sealing packaging blank or packaging, such as bags or bows,having at least one heat-sealed seam.

REFERENCES Patent Publications

-   FI 126474 B-   FI 126092 B-   FI 63806-   EP 0 195 458 B1

1. A method of manufacturing a multilayered fibrous product whichcomprises at least two overlapping layers, the method comprising:providing in a web forming process: a first layer comprising naturalfibers, and a second layer comprising synthetic fibers or particles,wherein the first and the second layers are formed and joined togetherby foam forming, said first and said second layers being arranged inoverlapping relationship, and said second layer being capable ofconferring properties of heat-sealing to the fibrous product.
 2. Themethod according to claim 1, wherein the first layer is formed into aweb from a first slush having a first composition, and the second layeris applied onto the web formed by first layer from a second slush havinga second composition to form a multilayered web, said first compositionbeing different from said second composition.
 3. The method according toclaim 1, wherein the first and the second layers are formed and joinedtogether in a foam forming process using a multilayer headbox.
 4. Themethod according to claim 1, wherein: the first layer is formed into aweb having a first solids content, said web is subjected to drying toincrease the solids content of the web to provide a modified web havinga second solids content, and the second layer is applied onto themodified web.
 5. The method according to claim 1, wherein the firstlayer is formed into a web having a first solids content, said web issubjected to pressing in a press section, to cylinder drying in cylindersection, or to a combination thereof, in order to increase the solidscontent of the web to provide a modified web having a second solidscontent, and the second layer is applied onto the modified web.
 6. Themethod according to claim 4, wherein the modified web comprises a drymatter content of 15 to 35% by weight before the second layer is appliedupon the modified web.
 7. The method according to claim 4, wherein thesecond layer is applied onto the modified web in the form of a foam. 8.The method according to claim 2, wherein the multilayered web formed bysaid first and said second layers is dried in order to produce amultilayered fibrous product.
 9. The method according to claim 8,wherein the drying of the multilayered web is carried out by non-contactdrying.
 10. (canceled)
 11. The method according to claim 1, wherein thefirst layer is formed from a first slush which contains natural fibersselected from cellulose fibers, lignocellulose fibers, and combinationsthereof.
 12. The method according to claim 1, wherein the second layeris formed from a second slush which comprises thermoplastic fibers orparticles selected from polylactide, glycolic acid polymer, polyolefin,polyethylene terephthalate, polyester, polyamide, polyvinyl alcohol,polybutylene succinate or bicomponent (bico) fibers or particles, andcombinations thereof.
 13. The method according to claim 1, wherein thesecond layer is formed from a second slush which comprises thermoplasticfibers having a linear density of 1-5 dtex, and an average length of1-50 mm.
 14. The method according to claim 1, wherein the first slushfurther comprises 0.1 to 15% by weight, calculated from the solidscontent thereof, of an additive selected from surfactants, bindingagents and combinations thereof.
 15. The method according to claim 1,wherein the second layer is formed from a second slush which, inaddition to the synthetic fibers or particles, further comprises 0.1 to15% by weight, calculated from the solids content, of an additiveselected from surfactants, binding agents, and combinations thereof. 16.The method according to claim 2, wherein the first or the second slushor both comprise at least one binder selected from the group consistingof natural binders and biopolymers, synthetic binders, copolymers,thermoplastic binders, and mixtures thereof.
 17. The method according toclaim 1, wherein the first or the second layer, or both, is impregnatedwith a binder.
 18. The method according to claim 1, wherein the firstlayer comprises more than 50% and up to 100% by weight, calculated fromthe total weight of the fibers of the first layer, of natural fibers.19. The method according to claim 1, wherein the second layer comprises50-100% by weight, calculated from the total weight of the fibers of thesecond layer, of thermoplastic fibers and/or particles.
 20. Amultilayered fibrous product comprising: a first layer comprisingnatural fibers, a second layer comprising synthetic fibers or particles,said first and second layers formed and joined together by foam forming,said first and said second layers being arranged in overlappingrelationship, and said second layer being capable of conferringproperties of heat-sealing to the fibrous product.
 21. The productaccording to claim 20, wherein the second layer comprises biodegradablethermoplastic fibers having a linear density of 1-5 dtex and an averagelength of 1-50 mm.
 22. The product according to claim 20, wherein thegrammage of the heat-sealing layer is 10 g/m² or less while the grammageof the first layer is 20-100 g/m².
 23. The product according to any ofclaim 20, wherein on an opposite surface of the second layer, theproduct further comprises a fibrous layer having hydrophobic properties.24. The product according to claim 20, wherein both the first layer andthe second heat-sealing layer further comprise the same thermoplasticbinder evenly spread upon or in the layers.
 25. A The product accordingto claim 20, comprising at least one sheet of the multilayered productfolded to form a package, wherein the product comprises at least oneseam zone in an area of which the multilayered product is attached toitself or to another sheet of the multilayered product by heat-sealing.26. (canceled)
 27. The method according to claim 1, wherein thesynthetic fibers or particles comprise thermoplastic fibers orparticles.
 28. The product according to claim 20, wherein the syntheticfibers or particles comprise thermoplastic fibers or particles.